Download a specific Blender source version from Git
You can check out the versions Tags and commit hashes here:
https://git.blender.org/gitweb/gitweb.cgi/blender.git/tags


git clone git://git.blender.org/blender.git (and be patient as
this is a monster repo!)
cd blender
git checkout v2.79 (or whatever version you need)


You can also fetch only a specific tag from blenger.org however, not sure that will save you much ban(note that the blender git repo does allow only fetch from tagged or head branch commits, no arbitrary commit from hash) following those instructions:
https://stackoverflow.com/questions/3489173/how-to-clone-git-repository-with-specific-revision-changeset

All that said, good luck with the build!